Bioscientifica Proceedings

Although cryopreservation of boar semen for artificial insemination (Al) was developed 35 years ago, cryopreservation conditions and Al strategies are still considered sub-optimal. Al with excessive numbers of frozenthawed sperm (5-6 x 10⁹ cells), still does not achieve fertility levels similar to Al using liquid semen because of reduced sperm survival. Frozenthawed (FT) spermatozoa have therefore not been the preferred option for commercial breeding programmes. However, substantial progress has been made regarding boar sperm cryopreservation. Adjustment of cooling and re-warming rates to biophysical properties of boar spermatozoa, new sperm package systems and the achievement of accurately consistent freezing of large numbers of samples using programmable freezers have contributed to post-thaw survival rates above 50%, a threshold similar to that used for bull AI-semen. Moreover, these post-thaw sperm survival rates are consistent within a large population of boars selected for sperm freezability potential, as occurs with Al-bull sires. When such post-thaw boar semen is deposited intra-utero, acceptable fertility (in terms of farrowing rates and litter size) is obtained. Currently, the most effective application of FT-semen for Al is achieved using deep uterine-Al (DUI) which allows placement of a minimal semen dose (in volume 0.5 to 10 mL and sperm number 0.5 to 1 x10⁹ total spermatozoa) into the anterior 1/3 of one uterine horn, with levels of fertility close to Al with liquid semen. However, owing to their shorter life span, FT-boar spermatozoa require an AI-to-ovulation interval not longer than 4-6 h, making peri-ovulatory Al a pre-requisite to obtain the highest possible fertility. Spontaneous ovulation most often occurs when two-thirds of oestrus has passed. Estimation of the duration of oestrus, taking into account the weaning-to-oestrus interval, is helpful when establishing appropriate AI-schedules. However, as the length of oestrus varies within and between farms, different Al strategies should be established a priori. The development of bio-sensors for spontaneous ovulation will widen the use of Al with frozen-thawed frozen semen.

© 2005 Society for Reproduction and Fertility